Carbonaceous materials are used in various applications such as conductive materials, capacitor electrodes, condenser electrodes, storage battery electrodes, nonaqueous electrolyte secondary battery electrodes, etc. and are fields and materials expected to be further developed in the future. These carbonaceous materials are conventionally manufactured from raw materials such as coconut shell, coal coke, coal or petroleum pitch, and furan resin or phenolic resin. It is recently expected that the use of fossil fuel resources becomes difficult in the future due to influences on the global environment and a price rise due to a decrease in amount of deposit.
Therefore, carbonaceous materials manufactured by using natural materials as raw materials are attracting attention as materials friendly to the global environment. However, natural materials contain various metals necessary for maintaining life activities of organisms. Therefore, if a naturally-derived carbonaceous material is used for an electronic material, such metal may act as impurities, leading to occurrence of an electric failure. Specifically, using the material for a capacitor or a storage battery causes problems, such as a short circuit due to the metal eluted and precipitated in the process of charging and discharging, or an increased internal resistance and a reduced capacity due to occurrence of a side reaction with an electrolytic solution etc. based on metallic impurities in the carbonaceous material. However, few technological developments have been made on methods of actively removing and refining metal derived from plants.
Under such a situation, a method is proposed in Japanese Laid-Open Patent Publication No. 2008-273816 (Patent Document 1) for refining a carbide together with a mineral acid such as hydrochloric acid or a base such as sodium hydroxide, and a method is proposed in International Publication No. 2014-038491 (Patent Document 2) for removing potassium and calcium by performing a demineralization treatment of organic substances derived from plants such as coffee beans and coconut shells in an acidic solution before detarring.
However, the method proposed in Patent Document 1 is a method of carbonizing a plant-derived material at 800° C. to 1400° C. and then treating the obtained carbide with an acid or an alkali, so that a metal component combines with carbon during carbonization and therefore is not sufficiently removed. Additionally, in Patent Document 1, a highly corrosive poisonous substance, i.e., hydrofluoric acid, is added in an excessive amount to a silicon compound to remove silicon after combination with carbon. However, hydrofluoric acid has an insufficient effect of removing magnesium and calcium and hardly removes phosphorus. The method of Patent Document 1 also has a problem that impurities in plant raw materials vary in content depending on the season and region and are difficult to smooth for industrial raw materials.
In the method proposed in Patent Document 2, if coconut shell is used for a plant-derived organic substance, the obtained carbonaceous material has a large amount of absorbed moisture. For example, using such a carbonaceous material for an electrode of a lithium ion battery causes problems such as a reduction in absorbed amount of lithium ions into the carbonaceous material and an increased possibility of self-discharge due to reaction of the adsorbed moisture with nitrogen atoms of the carbonaceous material and reaction of the adsorbed moisture with lithium ions.